pinctrl: sunxi: always look for apb block

The Allwinner pinctrl device tree binding suggests that a clock named
"apb" would drive the pin controller IP. However (for legacy reasons) we
rely on this clock actually being the first clock defined.
Since named clocks can be in any order, let's explicitly check for a
clock called "apb" if there is more than one clock referenced.

Kudo to Maxime for suggesting this much more elegant approach.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>